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Aquaculture 286 (2009) 154–155
Contents lists available at ScienceDirect
Aquaculture
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / a q u a - o n l i n e
Letter to the Editor
Comment on Beamish et al. (2007) “A proposed life history strategy for the salmon
louse, Lepeophtheirus salmonis in the subarctic Pacific”
L.M. Dill ⁎, C.J.C. Losos, B.M. Connors, P. Mages
Behavioural Ecology Research Group, Department of Biological Sciences, Simon Fraser University, Burnaby, BC, Canada V5A 1S6
a r t i c l e
i n f o
Article history:
Received 30 July 2008
Received in revised form 21 September 2008
Accepted 22 September 2008
A marine parasite on an anadromous host has a unique transmission challenge. It must infect a subsequent generation (or an alternate
host species) prior to its current host's entry into freshwater, where
the parasite (and often its host) will die. The salmon louse, a salmonidspecific parasite, faces just such a problem, and we would expect this
species to have evolved a strategy to maximize transmission during
the limited window of opportunity when incoming adults and
outmigrating juveniles (or alternate hosts) are present in an area
simultaneously.
In their 2007 paper “A proposed life history strategy for the salmon
louse, Lepeophtheirus salmonis in the subarctic Pacific”, Beamish et al.
(hereafter BEA) hypothesize a strategy enabling sea lice to solve this
problem. The data in this paper are interesting in showing that adult
Pacific salmon carry salmon lice into the coastal environment during
their migration and overlap with juveniles in the late summer
(August). Although these data confirm anecdotal evidence of the
presence of sea lice on adult salmon in the marine coastal
environment, something known to all fishermen, we argue that
BEA's hypothesis is fundamentally flawed and their paper invites
misleading citation. BEA use incorrect terminology and vague
definitions, and omit important literature while arriving at highly
speculative and, in our opinion, inappropriate conclusions.
While natural selection has undoubtedly shaped the life history
strategy of L. salmonis BEA claim that “the transport of sea lice into
coastal areas is a strategy employed by L. salmonis to improve their
productivity….” There are several fundamental problems with this
statement. The term “life history strategy” refers to the allocation of
individual resources to competing life functions (see, e.g., Winemiller
and Rose, 1992), something which inevitably involves trade-offs, for
example between growth and reproduction. BEA fail to identify any
such trade-offs and provide no support for their assertion that this
⁎ Corresponding author.
E-mail address: ldill@sfu.ca (L.M. Dill).
0044-8486/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.aquaculture.2008.09.024
attribute of the louse life history is the result of selection. Indeed,
“transport” per se cannot be a strategy or an adaptation—in fact, it may
be more reasonable to assume that this is simply a consequence of
parasitizing a migratory host. Only if it were shown, for example, that
lice delayed their production of offspring until host cohorts overlapped, so as to maximize transmission, would it be proper to speak of
a strategy.
BEA show that lice-infested adults are sympatric with “juvenile”
pink (Oncorhynchus gorbuscha) and chum (Oncorhynchus keta) salmon
in “coastal” areas in August. While it is reasonable to assume that
inter-generational transfer of lice may occur then, and this has been
shown by other studies (Krkosek et al., 2007), this cannot explain how
juvenile pink and chum salmon fry entering the marine environment
much earlier in the spring (March–early April) become infested with
lice, as these adults would not be present. As a result of the summer
bias in their data, there is no reasonable way the authors can make the
generalization that “some juvenile Pacific salmon would be infected
by resident salmon almost immediately after they entered the ocean”.
No data are provided to support this vague claim, and other studies
(discussed below) repeatedly show that lice prevalence and abundance on newly emerged fry are in fact very low.
To deal with this problem, BEA postulate that “Infected juvenile
salmon would also carry the parasite over winter in coastal areas
where the offspring of these sea lice can infect juvenile Pacific salmon
when they enter the ocean in the spring,” but once again provide no
data to support this. The authors claim “that large abundances of
juvenile Pacific salmon coexist with these returning adult Pacific
salmon. Most of these juvenile salmon will migrate into the open
ocean, but MANY (emphasis ours) coho (Oncorhynchus kisutch) and
chinook (Oncorhynchus tshawytscha) remain in the coastal areas
during winter.” BEA base this statement on a sample of 10 coho and 25
chum salmon captured in April–May in a very large area “in the
vicinity of Broughton Island and in Knight and Kingcome Inlets”.
Without knowledge of the region, a naïve reader could not know that
this spans from natal streams, all the way to the Queen Charlotte
Author's personal copy
Letter to the Editor
Strait–across both “nearshore” and “coastal” ecosystems, as defined by
the authors–and encompasses several hundred square kilometers. A
sample size of 35 fish during 2 months in a single year does not easily
allow for confident generalizations, especially over such a large area
with such vague descriptions. From this information, one cannot
determine whether the fish were captured in coastal or nearshore
areas—critical information to BEA's interpretation. Also of note is that
species and stage of lice were not determined for these fish, which is of
key importance in determining their origin and role in the transmission dynamics of the parasite.
Most importantly, high levels of infestation on very young fish in
the eastern Pacific have been reported only in areas with extensive
salmon aquaculture, such as the Broughton Archipelago (Morton and
Williams, 2004; Morton et al., 2004, 2005; Krkosek et al., 2005, 2006).
The existing literature on juvenile Pacific salmon in the nearshore
environment points to very low infestation rates in the first months
after seawater entry in areas without salmon farms. BEA refer to the
work of Wertheimer et al. (2003) as evidence to support infection of
salmon in areas without farms. However, BEA fail to mention any of
the life stages sampled by these authors, and they conveniently omit
the data presented by Wertheimer et al. on juvenile chum salmon
reared in seawater netpens in littoral areas for as many as 93 days
prior to release (May–early June) without any sign of infection. In late
June, when juvenile salmon were first encountered in neritic waters
(the equivalent of BEA's “coastal” waters), no evidence of infection was
observed on juvenile pink and chum salmon. It was not until July that
sea lice were observed on juvenile pink and chum salmon (at a
prevalence of 4.6% and 10.1%, respectively) concurrent with the return
and mixing of infected adults of the same species.
A study performed in B.C. found no evidence of infestation on
juvenile pink and chum salmon in either Smith (June to July) or Rivers
(mid May to July) Inlets (Morton et al., 2004). In the same study, very
low lice abundance was found in samples obtained near Prince Rupert
(2 L. salmonis on 566 individuals, May to June). None of these locations
contain fish farms. In addition, a three-year sampling program in
Chatham Sound on the north coast of British Columbia has
documented very low abundance of L. salmonis on juvenile pink and
chum salmon during April, May and June (Krkosek et al., 2007). It is
not until July, when returning adult chinook salmon occur in
sympatry with juvenile pink salmon, that L. salmonis prevalence
rises (from 2–3% to 50%). The important point in these findings, and
the one that argues most strongly against BEA's hypothesis, is that if
the route of transmission to juvenile salmon results from a naturally
evolved strategy, then the same patterns of infection should be seen
coast-wide. However, such patterns are seen only in the vicinity of fish
farms.
Although it is likely that some low-level transmission to newly
emerged salmon occurs from overwintering wild Pacific sub-adults,
these numbers must be dwarfed by the millions of farmed Atlantic
salmon present in the Broughton Archipelago, carrying infective lice,
as juvenile salmon migrate past them on their way out to sea (Orr,
2007). These farms have been identified as point sources of sea louse
infection on juvenile pink and chum salmon (Krkosek et al., 2005,
2006). Farmed salmon have similarly been implicated as a source of
infection for wild fish elsewhere (Tully and Whelan, 1993; Tully et al.,
1999; Bjørn et al., 2001; Bjørn and Finstad, 2002; Butler, 2002; Heuch
et al., 2005). BEA's paper is curious in failing to mention these farm
155
hosts in the Broughton Archipelago, despite this being the only place
on the coast where newly emerged wild fry are heavily parasitized.
In summary, BEA's errors of omission and their selective use of
their own and others' data lead the naïve reader to a conclusion that
cannot be substantiated. Their “conclusion” that the “transport of L.
salmonis into coastal areas is an evolutionary adaptation” is unwarranted and, indeed, is not a conclusion at all. In fact, the presence of
farmed salmon along the migration routes of very young wild salmon
represents an anthropogenic perturbation to a natural host–parasite
system that arguably makes it impossible to study the parasite's
evolved “strategy”, despite the authors' claim that their study is
intended to provide “an understanding of the natural production of
sea lice.”
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